1. Field of the Invention
The present invention relates to the treatment of metal bearing mineral materials for the extraction of desired metal values, such as silver, copper, zinc, arsenic and iron. The treatment may be for direct recovery of the desired metal values and/or for the enrichment in the solids of other constituents such as lead, sulfur, antimony, bismuth and gold. More specifically, the present invention relates to the leaching of such metal bearing mineral materials with an aqueous liquid containing sodium nitrite.
2. Description of the Prior Art
Because of present day emphasis on minimizing pollution of the earth and the atmosphere and because of more acute awareness of energy conservation as an industrial reality, metal recovery processes which involve ore roasting techniques have become less desirable. As an alternative, the lixiviation or leaching of metal bearing mineral materials by hydrometallurgical techniques is considered more appropriate since roasting gases, in which pollutants such as sulfur dioxide predominate, are not normally formed.
For instance, techniques are known for extracting silver and copper from silver and copper bearing solids by hydrometallurgical techniques such as by utilizing pressure oxidation in a nitric/sulfuric acid environment. For instance, see U.S. Pat. No. 4,084,961 to Caldon.
However, even ore leaching techniques, as so far provided by the art, suffer from certain drawbacks. Among these is the attendant expensive consumption of leaching agents, especially acid and oxidant, i.e. where soluble salts are to be formed of the desired metal values by the action of an oxidizing acid, such as nitric acid, and oxygen on the metal bearing mineral material. In order to obtain more efficient recovery of some desired metal values, the ore is sometimes pretreated to remove unwanted constituents. Where such pretreatment involves roasting steps, the above stated disadvantages must be similarly considered. Where such pretreatment involves hydrometallurgical steps, expensive consumption of treatment agents and problems of their disposal are often involved which when added to the energy and equipment requirements significantly detract from the overall economy of the system used. Moreover, once the desired metal values have been leached from the mineral material, for instance by an acid leaching treatment, the metal values must be separately recovered and the remaining leaching solution containing acid values and perhaps residual metal values must be disposed of or regenerated for recycling. Such recovery and work up add to the overall treatment cost.
In particular, one obvious disadvantage of acid treating under pressure a metal containing material, whether sulfide, oxide or mixture, in which any of the desired metals are associated with carbonates, is the substantial pressure developed by the release of carbon dioxide gas. Carbon dioxide always develops in such instances where an acid reacts with a carbonate material. Very often metal mines yield ores, particularly those involving silver, copper, zinc, arsenic, iron, lead, sulfur, antimony, bismuth, gold, and the like, in which the valuable metals are associated geologically with manganese or iron carbonate. In the flotation treatment of these ores, metals combined as carbonates very often concentrate with the sulfide or oxide flotation product.
Hence, the treatment of such flotation concentrates or the like by acid leaching under oxygen pressure in closed pressure vessels results in the need for additional oxygen pressure sufficient to counterbalance the partial pressure generated by released carbon dioxide gas. The end result is more elaborate leaching equipment, slower reaction rates, incomplete leaching as well as increased consumption of acid.
In addition, rapid and exothermic reactions are experienced in the nitric/sulfuric acid pressure leaching process, making the process difficult to control on a large scale. Particularly troublesome are the pressure spikes associated with an overall pressure increase.
The present invention has been developed with a view to providing an improved process for extracting metal values from metal bearing mineral materials.